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That's a good point mike, but I let the boat run bc it literally will play for hours. On Tuesday night when the alternator let go, the boat ran the entire time the stereo played probably about 6 hours and it pulled the batteries down and the 70 amps max that that thing could put out was not enough to keep them charged. before, the voltage while running and playing the boat would start out at about 14 Volts, then once played for a while. The voltage would start to come down And hang around 12 so I'd turn the stereo down and let the alternator charge them back up. Now that thing is a monster. It can hold a solid 13.5 volts on my digital voltage gauge and it never fell off last night.

That's a good point mike, but I let the boat run bc it literally will play for hours. On Tuesday night when the alternator let go, the boat ran the entire time the stereo played probably about 6 hours and it pulled the batteries down and the 70 amps max that that thing could put out was not enough to keep them charged. before, the voltage while running and playing the boat would start out at about 14 Volts, then once played for a while. The voltage would start to come down And hang around 12 so I'd turn the stereo down and let the alternator charge them back up. Now that thing is a monster. It can hold a solid 13.5 volts on my digital voltage gauge and it never fell off last night.

Bro honestly letting the boat idle isnt going to help much, an alternator is really barley putting any power out at idle.
It needs to be at 2K RPM to reach max output.

SO if you try the same thing with your new alternator their is a good chance you will toast it in time as well.

NOt trying to rag bro honest. I like your upgrade, and its a value added one but if you tie up and crank that long, you need to look into golf cart battery upgrades.

Mike are rated at 20amp for 570 minutes each
70amps for 180 minutes each <if i recall>

I have ran mine all day with the tunes about 1/2 and never an issue but i do have to shore charge them because my boat cant get them to max that fast

NEVER, EVER run your boat while at rest to get a charge when listening to the stereo.
Alcohol consumption amplifies the poisonous impact of carbon monoxide. Combined, it takes much less of both to kill someone. Add in stagnate air and you have an even greater risk. Several years ago two people died in Oklahoma from this exact practice. They passed out, fell off the swim platform when no one noticed and silently drowned.

You can have as many batteries as you want. Fully charged batteries represent no extra load to the alternator. However, and this is a big 'however', discharged batteries are a liability rather than an asset. A number of highly discharged batteries or an inordinately large battery capacity that is highly discharged combined with the stereo draw will have an alternator running as hot as an exhaust manifold. And btw, as Mike mentioned, an alternator at idle doesn't produce very much current. What can be worse is when exhausted batteries are used beyond their lifespan. This is extremely stressful for an alternator and will lead to an early alternator demise. If everyone only discharged their batteries no lower than 12.0 volts and batteries were replaced when needed and on time, alternator rebuilders would be put out of business.
If you have a large system that you play at rest for a protracted period then it is not practical to burn a tank of gas to fully recharge. You must have SMART AC shore charging to fully restore, not to mention fully condition.

Bro honestly letting the boat idle isnt going to help much, an alternator is really barley putting any power out at idle.
It needs to be at 2K RPM to reach max output.

SO if you try the same thing with your new alternator their is a good chance you will toast it in time as well.

NOt trying to rag bro honest. I like your upgrade, and its a value added one but if you tie up and crank that long, you need to look into golf cart battery upgrades.

Mike are rated at 20amp for 570 minutes each
70amps for 180 minutes each <if i recall>

I have ran mine all day with the tunes about 1/2 and never an issue but i do have to shore charge them because my boat cant get them to max that fast

I’m not sure I completely understand. Here’s my thought process:

The alternator converts energy from mechanical to electrical. The mechanical energy is supplied from the engine, whose input is chemical energy in the form of gasoline. The electrical output power of the alternator must be less than the mechanical input power from the engine, which must be less than the chemical power from gasoline combustion.

If the engine is idling under light load, the regulator will adjust the field current for the needed load power w/ constant voltage, approx. 13.8V. If the load is then increased, the voltage out momentarily droops, so the field current is raised by the regulator. The voltage at the alternator output increases to the right value, & the new current value is obtained. But the power has increased as well. This increase in electrical power mandates a corresponding increase in mechanical power from the engine.

If the fuel consumption is constant, i.e. no increase in fuel, than the idle speed will drop. Since the fuel consumed at the original engine idle speed did not increase, & the alternator demands more mechanical power from the engine, the engine must lose some of its mechanical power. The rpm drops as a result but the power out remains the same. Not sure how more engine rpms will make a difference unless your alternator is causing your engine to stall.

I would have to imagine that your factory alternator is sized such that it’s max output is enough to keep up with all demand even at idle. Add more demand though and I would say no matter how many batteries you have if your alternator is undersized ie max output power can’t keep up with demand you are in trouble as no matter the rpm it’s going to be running all out to keep up?

I’m in the same “boat” as you guys hope I don’t need to replace the alternator?

Re: Alternator took a dump last night

Well guys, I don't let it idle. It's at about 1000 rpm. Holds 13.5 volts solid and my buddies supra does the same all the time too. He actually runs the same alternator I do. We haven't ever had problems out of his. (Going on 4 summers now) our other boat I did has two summers under its belt with the same setup. I just did this to show guys if you want to run a bigger alternator its pretty easy. The supra was easier. Lol